1. Field of the Invention
This invention relates to an image forming apparatus utilizing the electrophotographic process or the like, and particularly to an apparatus for charging a recording material before transfer.
2. Description of the Related Art
Various processes such as the electrophotographic process, the heat transfer process and the ink jet process have heretofore been adopted in image forming apparatuses. Of these, an image forming apparatus using the electrophotographic process has advantages in high speed, high quality of image and quietude.
FIG. 3 of the accompanying drawings schematically shows the construction of an example of the image forming part of a conventional image forming apparatus using the electrophotographic process.
The image forming apparatus of the electrophotographic type has, for example, a drum-shaped electrophotographic photosensitive body, i.e., a photosensitive drum 1, as an image bearing body, and uniformly charges the surface of the rotating photosensitive drum 1 by primary charging means 2, and thereafter effects exposure 12 in accordance with image information by exposing means 11 such as an LED or a laser to thereby form an electrostatic latent image on the surface of the photosensitive drum 1. Thereafter, the electrostatic latent image is developed by a developing apparatus 8 by the use of a developer (a toner, or a toner with a carrier), and the toner is made to electrostatically adhere to the latent image, which is thus visualized as a toner image.
In synchronism with the formation of the toner image on such a photosensitive drum 1, a transferring material P is conveyed from a sheet feeding cassette 15 by conveying means 14, and the toner image on the photosensitive drum 1 is electrostatically transferred onto the transferring material P conveyed to a transferring position opposed to the photosensitive drum 1 by the conveying means 14, by the action of transfer charging means 4. Thereafter, the toner image transferred onto the transferring material P is fixed by being heated and pressurized by a fixing apparatus 21, whereby a permanent image is obtained on the transferring material P.
On the other hand, any untransferred toner residual on the photosensitive drum 1 after transfer is removed by a cleaning blade provided in a drum cleaner 10, and is collected into the container portion (waste toner container part) of the cleaner 10. The photosensitive drum 1 having had its surface thus cleaned is repetitively used for image formation.
Now, in recent years, color image forming apparatuses using the electrophotographic process have spread. There are various types of color image forming apparatuses. Besides the well-known multiple transfer type and intermediate transferring member type, there are the multiple developing type in which development is repeated on the surface of an image bearing body to thereby superimpose and form toner images of plural colors, whereafter the toner images are collectively transferred, and the in-line type which has image forming means (process stations) of a plurality of different colors along a transferring material conveying belt and toner images of the plural colors are superimposed and transferred to a transferring material conveyed by the conveying belt.
The color image forming apparatus of the in-line type can be made higher in speed and has many advantages including the advantage in quality of image due to the low frequency of toner image transfer. In this in-line type, there has also been proposed a construction in which for the improvement in usability and the reduction in installation area, the process stations are arranged in a vertical direction and a transferring material is conveyed substantially vertically.
FIG. 4 of the accompanying drawings shows an example of the construction of a full color image forming apparatus of the conventional in-line type. This apparatus has an electrostatic sucking belt, i.e., a conveying belt 14, as a transferring material conveying member, and the conveying belt 14 is passed over a drive roller 23, a sucking opposed roller 22 and tension rollers 13a, 13b. The conveying belt 14 is rotatively driven in the direction of arrow by the drive roller 23.
Process stations 31Y, 31M, 31C and 31Bk which are yellow (Y) magenta (M), cyan (C) and black (Bk) image forming parts are disposed along the peripheral surface of the conveying belt 14, and a transferring material is sequentially conveyed to the respective process stations by the conveying belt 14. Each process station 31 (31Y, 31M, 31C, 31Bk) has a photosensitive drum 1 (1Y, 1M, 1C, 1Bk), a primary charger 2 (2Y, 2M, 2C, 2Bk), a developing apparatus 8 (8Y, 8M, 8C, 8Bk) and a drum cleaner 10 (10Y, 10M, 10C, 10Bk), and the photosensitive drum 1, the charger 2, the developing apparatus 8 and the drum cleaner 10 are integrally made into a process cartridge which is made detachably mountable with respect to the main body of the image forming apparatus.
A transferring blade 4 (4Y, 4M, 4C, 4Bk) which is transfer charging means abuts against the photosensitive drum 1 with the conveying belt 14 interposed therebetween, and during the transfer of a toner image on the photosensitive drum 1 to the transferring material P, a transferring bias voltage is applied to the transferring blade 4 from a transferring bias power supply 32 (32Y, 32M, 32C, 32Bk) connected thereto.
When in the foregoing, an organic semiconductive electrophotographic photosensitive member (OPC photosensitive member) of the negative polarity is used as the photosensitive drum 1 and the exposed portion of a latent image in which negative charges have been attenuated by the exposure of the photosensitive drum 1 is to be developed, use is made of a developer including a toner of the negative polarity. Accordingly, a transferring bias voltage of the positive polarity is applied from a transferring bias power supply 32 to the transferring blade 4.
The transferring material P is conveyed from a sheet feeding cassette 15 toward the image forming part by a pickup roller 16 and sheet feeding rollers 17 and 18, and is once nipped by and between a pair of registration rollers 19a and 19b which are roller-shaped synchronous rotary members, and thereafter is supplied to the transferring material sucking part of the conveying belt 14 by the pair of registration rollers 19 in synchronism with the image forming operation on the photosensitive drum 1.
In the sucking part, a sucking roller 20 as sucking charging means is installed in opposed relationship with the sucking opposed roller 22 with the conveying belt 14 interposed therebetween, and the conveying belt 14 and the transferring material P are adapted to be nipped by and between the sucking roller 20 and the opposed roller 22. A voltage (sucking bias voltage) is applied from a sucking bias power supply (high power supply), not shown, to the sucking roller 20, whereby sucking charges are imparted to the transferring material P, and the transferring material P to which the charges have been imparted polarizes the conveying belt 14, whereby the transferring material P is electrostatically sucked to the conveying belt 14.
The transferring material P sucked to the conveying belt 14 in this manner passes through the respective process stations in succession, and yellow, magenta, cyan and black toner images on the respective photosensitive drums 1 are successively superimposed and transferred onto the transferring material P. Thereafter, the transferring material P is separated from the conveying belt 14 and is conveyed to a fixing apparatus 21, where the fixing of the toner images of the four colors is effected, and a full color permanent image is thus obtained on the transferring material P. Any untransferred toner residual on the photosensitive drum 1 after the transfer is removed by the cleaning blade 9 (9Y, 9M, 9C, 9Bk) of the drum cleaner 10, and is collected into the container portion of the cleaner 10.
As the above-described conveying belt 14, use is made of resin film such as polyvinylidene fluoride resin (PVDF), ethylene tetrafluoride-ethylene copolymer resin (ETFE), polyimide, polyethylene terephthalate resin (PET) or polycarbonate having a thickness of 50 to 200 xcexcm and volume resistivity of the order of 109 to 1016 xcexa9cm, or a rubber sheet comprising a substrate layer of rubber such as ethylene-propylene-diene three component copolymer (EPDM) having a thickness of the order of 0.5 to 2 mm, and covered with a coating consisting, for example, of urethane rubber having fluorine resin such as polytetrafluoroethylene (PTFE) dispersed therein.
In general the conveying belt 14 does not bear toner images on its surface directly and therefore is little contaminated by the toners, but during the jam of the transferring material or during the adherence of fog toners to the non-image bearing portions of the photosensitive drums 1, or at the starting of a system in which a registration mark or a density detection pattern is directly formed on the conveying belt 14, and it is detected and used for image control, the toners adhere onto the conveying belt 14 and contaminate it.
So, a belt cleaner 24 is provided to remove the contaminative toners on the conveying belt 14. Alternatively, the cleaning process of applying a cleaning bias voltage opposite in polarity to the bias voltage during the transfer to the transferring blade 4 in each process station 31 to thereby shift the contamination toners on the conveying belt 14 to the photosensitive drum 1, and collecting such toners into the drum cleaner 10 is carried out.
FIG. 5 shows a conventional image forming apparatus of the intermediate transferring member type. This apparatus is a full color image forming apparatus provided with a plurality of process stations along an intermediate transferring belt as an intermediate transferring member. In FIG. 5, members with the same numerals as those in FIG. 4 corresponds to the same members.
In the image forming apparatus using the conveying belt, the toner image of each color formed in each process station is directly transferred to the transferring material, whereas in the image forming apparatus using the intermediate transferring belt 27, toner images of respective colors formed in respective process stations 31 (31Y, 31M, 31C, 31Bk) are once primary-transferred to the intermediate transferring belt 27 and are superimposed, whereafter the toner images are collectively secondary-transferred onto the transferring material P.
The intermediate transferring belt 27 is passed over a drive roller 23, a tension roller 13 and a secondary transferring opposed roller 28, and as the intermediate transferring belt 27, use is made of a belt made of synthetic resin or a belt made of rubber.
By a process similar to that described with reference to FIG. 4, image formation is effected at predetermined timing in the respective process stations 31, and toner images formed on the photosensitive drums 1 (1Y, 1M, 1C, 1Bk) are successively primary-transferred onto the intermediate transferring belt 27 in respective primary transferring parts opposed to the primary transferring blades 4 (4Y, 4M, 4C, 4Bk), to thereby form a full color image comprising toner images of the four colors, i.e., yellow, magenta, cyan and black superimposed one upon another, and the toner images of the four colors on the intermediate transferring belt 27 are collectively secondary-transferred onto the transferring material P supplied to the secondary transferring part of the intermediate transferring belt 27 at predetermined timing via sheet feeding rollers 17, 18, registration rollers 19, etc., by a secondary transferring roller 29. The transferring material P subjected to the transferring step is conveyed to the fixing apparatus 21, where it is heated and pressurized and the toner images are fixed as a permanent image on the surface of the transferring material.
In the image forming apparatus of the electrophotographic type as described above, requirements for a higher speed and a higher quality of image have been heightening year by year and further, similar requirements are also heightening for transferring materials differing in nature from generally popular copying paper such as a transparency for overhead projector (hereinafter referred to as OHT) or thick paper.
Particularly about a higher quality of image, it is one of the most important elements to optimize the transferring bias voltage in the transfer charging means in conformity with the nature of the transferring material. However, the optimum transferring bias voltage for respective transferring materials has been changed, for example, by the state of the fluctuation of the ambient environment (temperature and humidity) or the fluctuation of the moisture content of the transferring material itself, and has been a great hindrance to a higher quality of image.
As regards the achievement of a higher quality of image for transferring materials of high electrical resistance as typified by thick paper or the like, in this case, a high transferring bias voltage becomes necessary as compared with popular copying paper. However, to apply a high transferring bias voltage, not only a correspondingly expensive power supply becomes necessary, but also in the full color image forming apparatus of the tandem type as shown in FIG. 4, a greater bias voltage is required in a more downstream station and therefore, the destruction of the insulation of the photosensitive drums and the belt has posed a problem.
As one of means for solving such a problem, there is known, for example, Japanese Patent Application Laid-Open No. 6-27837 or Japanese Patent Application Laid-Open No. 11-161035.
Both of the techniques of these publications make it possible to pre-charge the transferring material on the conveying belt immediately before toner images are transferred to the transferring material, to thereby suppress the transferring bias voltage required for the transfer of the toner images to a low level.
In these techniques, however, the bias voltage for the pre-charging is primarily set depending on whether the transferring material is a high-resistance transferring material, and the differences between the states of individual transferring materials such as the kinds, moisture contents and surface resistance values of the transferring materials are not taken into account, and such techniques have been insufficient from the viewpoint of a higher quality of image.
Further, provision is not made of means for discriminating whether the transferring material is a high-resistance transfer material, and this has been left to the user""s judgement. Thick paper, OHT, etc. include what do not come under the category of high-resistance transferring material but yet are commercially available, and it becomes a factor which deteriorates the quality of image to effect the pre-charging for the high-resistance transferring material in case of image formation on such transferring materials.
Specifically, when pre-charging is effected by a predetermined bias voltage on low-resistance OHT or OHT greatly reduced in resistance under a high humidity environment, bad charging has sometimes occurred. That is, when OHT has become low in resistance due to its environment, the pre-charging bias voltage becomes excessively great to the OHT or charges once accumulated by the pre-charging escape halfway, and in the central portion wherein charges remain, the transferring bias voltage is too weak and the image becomes faint due to bad transfer, and in the end portions from which charges have escaped, the transferring bias voltage is too strong and leak occurs, and likewise the image becomes faint due to bad transfer. As the result, it has been found by the applicant""s investigation that as shown in FIG. 6 which shows the case of a solid image, a bad image of which the black portion assumes a doughnut-like shape (doughnut phenomenon) is caused.
There has also been proposed the technique of providing a current detecting mechanism for a high power supply circuit for applying the transferring bias voltage, applying a predetermined bias voltage during the non-supply of sheets or during the passage of the leading edge blank portion of a transferring material and detecting the then flowing current value to thereby detect the ambient environment or discriminate the electrical resistance value of the transferring material, and optimizing the transferring bias voltage on the basis of these.
However, the requirement for a higher speed has heightened simultaneously with the requirement for a higher quality of image and for this, it is necessary to increase the process speed. As the result, there arises the problem that the time during which the blank portion on the leading edge of the transferring material passes the transfer charging means, i.e., the time during which the discrimination of the electrical resistance value of the transferring material can be effected, becomes short and the accuracy of the discrimination lowers or wrong discrimination is effected and the mechanism does not effectively act for the higher quality of image which is the original purpose.
So, in Japanese Patent Application Laid-Open No. 6-35337, there is described the technique of discriminating the electrical resistance of a transferring material from the current value when a detection bias voltage is applied in a sucking portion. According to this, the discrimination of the electrical resistance value can be effected during the time from after the transferring material has come into a sucking part until it comes into a transferring part, and it seems that the time required for this can be sufficiently secured.
However, in order that the discrimination of the electrical resistance value may be accurately effected for a transferring material of a high electrical resistance value such as thick paper or OHT, it is preferable to stepwisely apply the detection bias voltage up to a reasonably high bias area. If a high bias voltage is stepwisely applied to such a transferring material, charges corresponding to the applied bias voltage will remain in terraces on the surface of the transferring material, and this will cause terraced unevenness in the transfer of a toner image in the transferring part. After all, to prevent this, the discrimination of the electrical resistance value must be effected in the blank portion on the leading edge of the transferring material, and like the discrimination in the transferring part, the discrimination in only the sucking part has been insufficient as a measure for a higher speed.
It is an object of the present invention to provide an image forming apparatus which can effect good transfer irrespective of transfer materials.
It is another object of the present invention to provide an image forming apparatus comprising an image bearing body bearing a toner image thereon, transferring means for transferring the toner image on the image bearing body to a transferring material, and charging means having a voltage applied thereto to thereby charge the transferring material before the transfer by the transferring means, wherein the voltage applied to the charging means is controlled on the basis of a current produced when the leading edge portion of the transferring material is charged by the charging means.
It is another object of the present invention to provide an image forming apparatus comprising an image bearing body bearing a toner image thereon, transferring means having a voltage applied thereto to thereby transfer the toner image on the image bearing body to a transferring material in a transferring part, charging means for charging the transferring material before the transfer by the transferring means, and detecting means for detecting a current produced when a voltage is applied to the charging means, wherein the voltage applied to the transferring means when the image-formed portion of the transferring material passes the transferring part is controlled on the basis of a current produced by a predetermined voltage being applied to the transferring means when the leading edge portion of the transferring material passes the transferring part, and the predetermined voltage is controlled on the basis of the output from the detecting means.
It is another object of the present invention to provide an image forming apparatus comprising an image bearing body bearing a toner image thereon, transferring means having a voltage applied thereto to thereby transfer the toner image on the image bearing body to a transferring material in a transferring part, charging means for charging the transferring material before the transfer by the transferring means, and detecting means for detecting a voltage produced when a current is applied to the charging means, wherein the voltage applied to the transferring means when the image-formed portion of the transferring material passes the transferring part is controlled on the basis of a current produced by a predetermined voltage being applied to the transferring means when the leading edge portion of the transferring material passes the transferring part, and the predetermined voltage is controlled on the basis of the output from the detecting means.
It is another object of the present invention to provide an image forming apparatus comprising an image bearing body bearing a toner image thereon, a transferring material bearing body bearing a transferring material thereon, transferring means for transferring the toner image on the image bearing body to the transferring material borne on the transferring material bearing body, charging means having a voltage applied thereto to thereby charge the transferring material borne on the transferring material bearing body in a charging part before the transfer by the transferring means, and detecting means for detecting a current produced when a voltage is applied to the transferring material bearing body on which the transferring material is not borne, wherein the voltage applied to the charging means when the transferring material passes the charging part is controlled on the basis of the output from the detecting means.
It is another object of the present invention to provide an image forming apparatus comprising an image bearing body bearing a toner image thereon, a transferring material bearing body bearing a transferring material thereon, transferring means for transferring the toner image on the image bearing body to the transferring material borne on the transferring material bearing body, charging means having a voltage applied thereto to thereby charge the transferring material borne on the transferring material bearing body in a charging part before the transfer by the transferring means, and detecting means for detecting a voltage produced when a current is applied to the transferring material bearing body on which the transferring material is not borne, wherein the voltage applied to the charging means when the transferring material passes the charging part is controlled on the basis of the output from the detecting means.
It is another object of the present invention to provide an image forming apparatus comprising an image bearing body bearing a toner image thereon, transferring means having a voltage applied thereto to thereby transfer the toner image on the image bearing body to a transferring material being conveyed in a transferring part, and charging means having a voltage applied thereto to thereby charge the transferring material in a charging part before the transfer by the transferring means, wherein when in the conveying direction of the transferring material, the length of the transferring material is greater than the distance from the charging part to the transferring part, the voltage applied to the transferring means is changed over at timing whereat the trailing edge portion of the transferring material passes the charging part.
Further objects of the present invention will become apparent from the following description.